This invention relates to mechanisms for drying using a microwave and vacuum process. More particularly, it relates to an improved drying chamber for use in drying pharmaceutical powders, granules, tablets and the like using a combination of microwave energy, product mixing and application of a vacuum.
The combined use of microwaves and a vacuum to dry various materials has become commonplace and has been applied in a variety of industries. Typical of the application of microwave drying to industrial uses is the disclosures of McKinney, U.S. Pat. No. 4,015,341 issued Apr. 5, 1977, for "Seed Drying Process and Apparatus", and Durant, U.S. Pat. No. 4,229,886 issued Oct. 28, 1980, for "Microwave Heated Vacuum Dryer for Powders". Microwave vacuum drying processes are also useful in the pharmaceutical industry, as described in the applicant's co-pending application Ser. No. 037610, filed Apr. 13, 1987, the disclosure of which is hereby incorporated by reference.
The microwave vacuum drying art, is however, still in its infancy. Devices capable of performing the drying function are often bulky, cumbersome and constructed with an excess of parts. In some instances, the shortcomings of the devices results in relatively inefficient equipment that is difficult to manufacture and use.
These drawbacks are particularly present in the designs for microwave vacuum drying chambers. Drying chambers must perform multiple functions. The chamber must provide a fully enclosed containment area for the microwaves that does not allow leakage of stray microwave energy. The chamber must be air tight, often for lengthy drying periods. In the pharmaceutical industry, chambers must usually include somemeans for mixing or rearranging the position of the product to be dried to prevent uneven product drying. The chamber must not allow condensation of water (or other product solvent) on its interior. In the case of powdered pharmaceuticals, such condensationcan cause the product to be "lumpy" or unevenly dried.
The chamber must also have numerous entry ports for sensors and other observation and monitoring devices, such as temperature, pressure or microwave field strength sensors, or ports for viewing the product during drying. The chamber should be easily cleaned, so that a variety of different pharmaceutical products can be dried within the chamber without any residue from a previous product left in the chamber to contaminate the subsequent batch. It is also desirable to mix, granulate and dry in a single chamber. The chamber should make loading and unloading of the product a task that does not require excessive time, labor or complicated procedures. Because of these requirements, many drying chambers used in the past, while effective in accomplishing their intended purposes, have not fully met all of the necessary criteria, due to the use of complicated configurations that increase the difficulty of manufacture, require more space than necessary, and are complicated to use.
Mixing or agitation of the product within the drying chamber has posed a number of difficult problems. The portion of the mixing mechanism operating within the drying chamber must normally be transparent to microwave radiation to prevent damage to the product and equipment from excessive energy concentration. The mixing mechanism must nevertheless be sufficiently sturdy and durable to move substantial volumes of product, if the apparatus is constructed to mix a commercially viable quantity of product. The mixing mechanism must also be capable of providing thorough mixing so that unevenly dried pockets of product are not produced. High shear mixing can damage the product being dried. Metal mixing elements provide the necessary strength for rapid mixing, but can damage the product and are not transparent to microwaves. Mixing elements made of synthetic materials, such as polypropylene or similar plastics, are gentler than metals and sufficiently transparent to microwaves. Such plastics, however, are easily scratched or damaged and are less capable of high volume mixing.
All of these criteria must be accomplished within a drying chamber having the limitations described above. Provision of a solution to these problems is therefore the intent of this invention.